German Patent Application No. 39 35 243 describes a conventional invertor circuit including a controlled rectifier, a DC link circuit, an invertor, and a battery. An alternating-current source is connected to the input of the controlled rectifier. The input AC voltage is converted into a DC voltage, whose value is controlled to a setpoint value by phase control of the thyristors in the controlled rectifier. The inverter converts this generated DC voltage into a predetermined AC voltage, which is supplied to a load. The DC voltage includes ripple voltage components which are produced by the rectification of the input AC voltage. These ripple voltage components are damped via an inductor and a capacitor in the DC link circuit, so that a DC voltage with a reduced ripple component passes to the inverter. The battery is connected in parallel with the DC voltage output of the controlled rectifier. In this state, the battery is charged in "floating" manner. However, if the charging current for the battery contains ripple components, then they produce heat in the battery, placing an additional stress on the battery. The inductor in the DC link circuit damps these ripple components to a defined magnitude. Such an inverter circuit can be used as a power supply unit which is protected against mains failure, also called an uninterruptable power supply (UPS).
There is now a contradiction in the design of the inductor. The inverter requires an inductor having low inductance while, in contrast, an inductor having high inductance is required while the battery is being charged, in order to suppress the ripple components. As described in German Patent Application No. 39 35 243, this discrepancy is solved by arranging a saturable inductor or an inductor having a device for suppressing the magnetic flux during charging in a supply lead to the battery.
Using this battery coupling apparatus, the battery is always connected to the DC link circuit of the inverter circuits (the battery is continuously loaded with alternating current). In addition, it is not possible to carry out controlled charging and discharging with an adjustable current for cycling purposes (battery maintenance).
A plurality of variants of a DC pulse-controlled converter are described in German Patent Application No. 31 04 965. Using the plurality of variants a battery can be charged from an AC mains, it being possible for the peak value of the mains AC voltage to be less than or greater than the voltage of the battery to be charged. The individual DC pulse-controlled converter variants differ by having different combinations repective series circuits of known step-up controllers and step-down controllers. Power transistors having a high switching frequency are used in each of the DC pulse-controlled converters. If required, frequency thyristors can also be used as an alternative for this purpose. The thyristors in the DC pulse-controlled converters are just switched at twice the mains frequency. As a result of the combination respective series connection of conventional step-up and step-down controllers, the DC pulse-controlled converters are of simple design, low weight and can be produced economically. However, it is disadvantageous that the energy flow is possible in only one direction, to be precise from the AC mains to the battery.
A step-up/step-down controller combination is described in the paper "Modelling the Three Phase Propulsion system of a Modern Multisystem Locomotive" by W. Geissler and F. Unger-Weber, printed in the EPE'91 Conference Proceedings, Florence, 4th Meeting, pages 4-632 to 4-637. This controller combination comprises two converter bridge arms which are each connected electrically in parallel with a capacitor and whose negative terminazls are connected to one another. In addition, an inductor is provided which connects the center terminals of the two converter bridge arms to one another. Thyristors which can be turned off and to each of which a freewheeling diode is assigned are provided as the active converter devices in both converter bridge arms. If the distribution line voltage (input voltage) is greater than the DC link voltage, then this controller acts as a step-down controller, it being possible to regulate the inductor current by pulsing an active converter device. If the DC link voltage is greater than the distribution line voltage, then this controller acts as a step-up controller, it likewise being possible to regulate the inductor current by pulsing an active converter device. The advantage of this controller circuit is that it is possible to alternate continuously between the step-down and step-up function. The disadvantage is that the thyristors which can be turned off and the associated freewheeling diodes have to carry the entire load current, as a result of which a high power loss always occurs in the semiconductor components used.